Market leaders change. Investing principles endure.
Is the US stock market overly reliant on just a few mega-cap firms? History suggests the leaderboard will change – and markets will adapt.
Key takeaways
- Market concentration isn't new. History shows that today's leaders are unlikely to remain dominant indefinitely.
- As market leaders change, new companies emerge to take their place, helping drive long-term equity market growth.
- Owning the whole market remains one of the most effective ways to capture future winners wherever they emerge.
A long-running rally, led by a handful of the largest companies, has prompted concerns that the US stock market is dependent on the fate of just a few mega-capitalisation companies. The central question is, “What happens when optimism about the current market leaders wanes?”
History suggests that the leaderboard will change – and the market will adapt.
Stock market concentration ebbs and flows: Leaders change across cycles
The largest companies naturally command attention, but their dominance has never been permanent. Consider how the market has evolved from the era of heavy-industry giants like Standard Oil of New Jersey (now Exxon) and General Motors to today’s technology leaders.
Change comes – even for the largest companies
Notes: Data are quarterly as of 31 December 2025. For details about our methodology, please refer to the notes at the bottom of this article.
Sources: Vanguard calculations using CRSP Research Data Products from Morningstar.
Changing economic conditions and the process of creative destruction lead to the growth and decline of companies and industries. In the late 1950s, for example, the US equity market saw levels of concentration like those of today, with the 10 largest companies accounting for roughly 32% of the market’s capitalisation. As of December 2025, the same companies made up just 2% of the market.
Conversely, today’s US stock market leaders entered the market with negligible weightings and have grown into dominant positions. Apple, for example, went public in 1980, accounting initially for just 0.1% of the market. It accounted for 6.7% as of December 2025, trailing only NVIDIA (7.6%).
The relationship between market concentration and long-term market performance
Concerns about stock market concentration may revolve around the assumption that the eventual decline of current leaders will derail long-term market returns. The historical record suggests otherwise.
As former leaders have given way to new innovators, their decline in weight has not led to a corresponding long-term decline of the US equity market. To the contrary, the emergence of today’s market leaders has filled the gap – a “handover” that has helped the US equity market to deliver long-term gains. Since 1958, the US equity market has delivered an annualised return of roughly 11%1. Even inflation-adjusted returns are strong, measuring approximately 7%2.
To be clear, investors have had to endure rough patches to capture those long-term gains.
The long-term growth of the US stock market
Notes: Data reflect quarterly total returns for the CRSP NYSE/NYSE American/NASDAQ/NYSE Arca Value-Weighted Market Index, as provided by CRSP, between 1958 and 2025. Bear markets are defined as periods in which US stocks decline 20% or more from the end of any given quarter. Past performance is no guarantee of future results. The performance of an index is not an exact representation of any particular investment, as you cannot invest directly in an index.
Sources: Vanguard calculations using CRSP Research Data Products from Morningstar.
The stock market reflects a changing economy
The market’s shifting composition is a feature, not a flaw. As industries rise and fall, investors adapt, causing the market to continuously reflect the structure of the economy. Technology companies’ current dominant share of the US market is striking but not entirely unprecedented. There have been periods in history when oil and industrial companies accounted for similar shares of the US stock market, notably in the 1940s and 1960s.
Economic changes shape stock market industry exposures
Notes: Data are quarterly as of 31 December 2025. For details about our methodology, please refer to the notes at the bottom of this article.
Sources: Vanguard calculations using CRSP Research Data Products from Morningstar and the Kenneth R. French Data Library.
Today’s winners may be tomorrow’s losers – but history suggests that equity markets will endure
At some point, the identity of the market’s largest companies will change. The adjustment may come with a sharp downturn, an extended period of subdued gains, or something in between.
It’s hard to know who tomorrow’s winners will be or when they’ll emerge, but if you own the entire market, chances are you already own them. Even when markets appear concentrated, history reinforces the value of diversified exposure to capture economic progress. Jack Bogle’s message of ‘owning the haystack’ remains as relevant today as it was 50 years ago.
Notes:
We identify the 1958 cohort of stocks by ranking them by their proportional percentage of total market capitalsation on 31 December 1957. We then track their corresponding quarterly percentage share of the total market capitalisation of the CRSP US stock market database over the subsequent years. If a stock is delisted or goes bankrupt, we assume the subsequent weight for that stock is 0%; if it is merged, we locate the successor company PERMCO (a unique permanent company identifier created by CRSP to track companies) in the CRSP dataset and then utilise the successor company weight for the forward periods. The one exception is AT&T, which split into several regional firms in the mid-1980s; in this case we only track the primary parent stock. We repeat this process for the 2025 cohort but track the market capitalisation share backward in time from 31 December 2025. We select 1958 because the S&P 500 Index was introduced in 1957, and our goal is to track the matching period of the S&P 500 Index. Market constituents come from the CRSP 1925 US stock market database. Data is filtered to include common stocks that have a primary listing on either the NYSE, AMEX/NYSE American or NASDAQ stock exchanges, are issued by US-incorporated corporate issuers, and that trade regular-way (a stock trading under standard settlement terms rather than special conditions) without a special designation. Stocks with multiple share classes are aggregated by PERMCO. Annualised returns are estimated based on quarterly total returns for the CRSP NYSE/NYSE American/NASDAQ/NYSE Arca Value-Weighted Market Index as provided by CRSP.
We map stocks to industries by linking each stock to a four-digit Standard Industrial Classification (SIC) code using CRSP data. We then use the 49 industry groups from the Kenneth R. French Data Library and link each stock’s four-digit SIC code to the corresponding industry in the Kenneth R. French dataset. We attempt to fill any stock-level data-mapping gaps through the following hierarchy: 1) backfill or forward-fill the stock’s SIC code from the adjacent quarter; 2) use a stock’s PERMCO identifier or use the SIC code for PERMCO peers in the same quarter where a stock is missing the SIC code; 3) backfill or forward-fill the SIC code for the PERMCO peer in adjacent quarters where the stock is missing the SIC code; and 4) classify the industry as unknown if the SIC code is still missing. Analysis begins with September 1926 based on data availability. Market constituents come from the CRSP 1925 US stock database. Data is filtered to include all common stocks that have a primary listing on either the NYSE, AMEX/NYSE American, or NASDAQ stock exchanges, are issued by US-incorporated corporate issuers, and that trade regular-way without a special designation. Stocks with multiple share classes are aggregated together by PERMCO. Data rounds to the nearest whole percentage point.
1 Annualised returns are estimated based on quarterly total returns for the CRSP NYSE/NYSE American/NASDAQ/NYSE Arca Value-Weighted Market Index as provided by CRSP.
2 Inflation-adjusted returns deflate the nominal returns by the quarterly change in the Consumer Price Index for All Urban Consumers: All Items in U.S. City Average (CPIAUCSL), which is obtained from the Federal Reserve Bank of St. Louis (https://fred.stlouisfed.org/series/CPIAUCSL). Sources: Vanguard calculations, using data from CRSP, as of 31 December 2025.
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